1. Field of the Invention
This invention relates to pressure sensing apparatus, and more particularly to pressure sensing apparatus employing a thin, pressure sensitive resistance wire.
2. Description of the Prior Art
There has been a continuing problem in the achievement of efficient and accurate pressure sensing over a wide pressure range, particularly for applications such as the detection of leaks in a vacuum system, in which a high level of sensitivity may be required. The most sensitive device commonly used for leak detection is a helium-leak detector, which is basically a mass spectrometer in which the readout is confined to the helium line. This is a known technique which is capable of producing highly reliable results in the pressure range below about 10.sup.-3 Torr, and down to about 10.sup.-11 Torr. Leak detection is accomplished according to this method by squirting the outside of suspected areas on the vacuum housing with a small blast of helium from a pressure tank. The helium passes through any leaks in the area and into the vacuum system, where it is detected by the mass spectrometer. A more detailed description of this technique, and also of the acetone-gauge technique described below, is provided in the text "Practical Vacuum Systems" by Roland Rutledge La Pelle, Chapter 17, published by McGraw Hill in 1972.
A major drawback of the mass spectrometer approach is the fact that it is generally inoperative at pressures greater than approximately 10.sup.-3 Torr. Many vacuum systems as originally assembled have so many leaks that they cannot be evacuated to a point permitting employment of the helium-leak detection method. Other undesirable aspects of mass spectrometers are their relative expense, the lengthy initial warm-up which may take up to an hour, and the fact that their electronics tend to saturate under gross leak conditions. After such saturation a lengthy delay in operation, as much as a half hour, may be required before the system can be returned to operation.
Another problem associated with leak detection in the mass spectrometer pressure range arises from the fact that residual gas may be left on the interior walls of the evacuated housing. As the gas slowly escapes from the walls, a "virtual leak" is present but may not be detected with a standard helium leak detector. A solution to this problem involves a fairly complex modification to the mass spectrometer providing a water vapor readout capability coupled with heating of localized areas on the vacuum housing to expel residual gas from the locally heated walls. If a pressure increase is detected, a residual rather than a real leak is indicated.
The acetone-gauge technique mentioned above is useful for leak detection in the approximate pressure range of 10.sup.-3 to 2.times.10.sup.-1 Torr. This technique involves spraying or painting a small amount of acetone onto a spot on the outside of an evacuated housing where a leak is suspected, and connecting a thermocouple gauge on the pump side of the system so that it reads manifold pressure. If a leak actually exists at the point where the acetone is applied, an immediate pressure rise will occur in the gauge, which then gradually returns to its former reading as the acetone evaporates and is expelled from the system. Once the larger leaks have been located and repaired in this manner, the achievement of a vacuum level low enough to permit use of the helium leak mass spectrometer technique has hopefully been attained.
Other leak detection techniques are known which are useful at pressures greater than 2.times.10.sup.-1 torr. In the audio amplification approach, which is useful up to approximately one atmosphere pressure, large leaks are detected by passing a sound pickup device over the housing, and listening to detect the sound of gas flowing into a leak. Another known technique which is useful in a more limited pressure range, between about 2.times.10.sup.-1 and 20 Torr, is the electrical glow discharge technique, which is applicable to glass, but not metal, vacuum housings.
Another limitation of many of the above pressure sensing techniques is that they are dependent upon the establishment of relatively large gas throughputs through the use of relatively large evacuating pumps. It would be desirable to be able to sense pressure with a small pumping system.
Another pressure sensing technique involves the use of pressure sensitive resistance wires, a form of thermistor. In such devices the wire is exposed to the pressure which is to be measured. The resistance of the wire is determined by its temperature, which in turn is dependent upon the temperature and thermal conductivity of the immediately surrounding gas and the ability of the gas to conduct heat away from the wire. Within a relatively small pressure range, up to approximately 1,000 Torr, changes in the gas pressure are accompanied by sometimes significant changes in the thermal conductivity of the gas, resulting from an increased or decreased density of the gas. A transient change in the gas temperature may also accompany pressure changes in accordance with the equation of state. Pressure indications may thus at least theoretically be obtained by using the wire to sense the thermal properties of the gas.
Pressure sensitive wires, however, have generally not been fully suitable for use in highly sensitive applications, such as vacuum leak detectors. Sensitivity is limited by the thickness of the wire, with sensitivity decreasing as the wire gets larger. Therefore, it is highly desirable that very thin wires by employed. For example, a wire thickness down to the order of 0.0001 inch is disclosed in U.S. Pat. No. 3,888,110 to Clark. The use of very thin wires, however, has led to a serious structural problem in that the wires are susceptible to being torn from their mountings under the sudden gas flows and pressure differentials caused by gross leaks. Various designs have been made which at least partially alleviate this problem, such as U.S. Pat. No. 1,768,415 to Matunaga. In this patent the resistance wire is encapsulated within an inner chamber in the vacuum device and isolated from direct contact with the gas of interest. While the wire is thus protected from physical impairment by gas dynamics within the vacuum device, its segregation from those gases may limit its accuracy. Other attempts to solve the problem may be found in U.S. Pat. No. 3,106,088 to Kieselback, in which the wire is surrounded by a perforated gas flow barrier; in U.S. Pat. No. 3,075,379 to Schmauch, in which the wire is disposed transversely to the gas flow between a pair of shielding barriers, and in U.S. Pat. No. 3,720,093 to Gill, in which a thermistor is shielded from direct gas flow by placing it in a cavity and surrounding it with a mesh thimble. In the latter two patents the shielding devices are intended primarily to shield the wire from direct gas flow in order to limit the cooling effect on the wire, and they are not specifically directed to a thin wire leak sensor application. In none of the above devices does the pressure sensitivity approach that of the mass spectrometer.
Accordingly, there is a need for a relatively inexpensive pressure sensing device which is sensitive at low pressures approaching a vacuum level, produces a useful output over a wide pressure range, and avoids the electronic saturation problems of mass spectrometers.